"Hello there," says Rupert Grint, "I'm Rupert Grint and ... I'm ... I'm ..." The Harry Potter star pauses, bites his lip and, deep in concentration, takes a swig of Coke. To his chagrin, he's been asked to recite the rap he recorded for a video aged 10, in which he outlined why he should be cast as Ron Weasley, the boy wizard's pal, in Harry Potter and the Philosopher's Stone, the first film in the global phenomenon. You might think the words would be etched into his brain given the part they played in securing him the role but, 10 years later, Grint can't recall a single line. "Erm ... right ... I'm Rupert Grint and I'm ... I'm ..." Mint? "No," he says unruffled. "And I ... I ... hope you don't think I stink." He laughs sheepishly. "Or something like that anyway."

Blame the actor's amnesia on fatigue. It is a muggy early evening in late June and Grint has been working since 7.15am at Leavesden Film Studios, a few miles north of Watford, on Harry Potter and the Deathly Hallows, the seventh and final tale of magic and mischief, which is set for release in 2010. Now, curled up on a sofa in his dressing room - home to a dartboard, a table tennis table and a small kitchen festooned with photographs of Grint and his co-stars engaged in youthful high jinks - he is chewing over the best hairstyles in pop, as befits a man who, two years ago, came fifth in Brylcreem's list of the best male film star mops.

Like his Harry Potter colleague Daniel Radcliffe (with whom he stars in the imminent new film, Harry Potter and the Half-Blood Prince), Grint is a huge fan of punk and its offshoots. "I love the madness behind it. The craziness, the passion. And the whole anarchist thing is quite cool." Naturally, he cherishes much of the music too - particularly the Pistols, the Dead Kennedys and the Clash. Just don't ask him to divulge his favourite punk albums.

"Well, I've got quite a few Clash ones, but ... er, I'm not very good with names. One, though, is something to do with rope. Give Em Enough Rope? Yeah, that's it. London Calling? Yeah, obviously I've got that one. Sandinista? Sandinista?" He shakes his head, nonplussed. "No, I've definitely not got that."

Now 20, Grint was a latecomer to purchasing CDs. He was raised in Watton-at-Stone, Hertfordshire, where he "performed a few raps in school" despite having very little interest in hip-hop apart from Afroman's Because I Got High, the chart-topping 2001 "comedy" single. Then, inspired by old photos of his mum, "who looked kinda punk and a bit 80s", Grint became obsessed with punk's iconography and Jamie Reid's images in particular.

"I think my love of the music comes from that," he says. "I own quite a few vintage Pistols T-shirts." But not, alas, the vest top famously sported by Sid Vicious depicting two gay cowboys exposing their penises. "No," he says, spluttering, "I haven't got that one." Wide-eyed, Grint smiles, as he does repeatedly, and reaches for his drink from a table feet away. "All the T-shirts I have are more, you know ... basic."

Punk aside, Grint's favourite genre is Britpop. Aged 15, he idolised Oasis and Blur, he says, plus Richard Hawley's former outfit the Longpigs and "some weird ones, like Swede ... er, sorry, no: Suede". Again the aesthetics proved as potent as the music: he loved Parklife's sleeve and the contents therein.

"I've bought albums simply because I like the look of the sleeve," he admits, "like the Dead Kennedys' Frankenchrist. And I've also done that because of funny band names, or funny song titles. I got the Kennedys' Kill the Poor for the title alone." The same is true of Mogwai, he says, whom he's been listening to on the Potter set. "I thought they sounded intriguing and decided to explore further. And they've got some great track names. There's one called The Sun Smells Too Loud." He unveils another winsome smile. "I love the sound of that."

Such delight in crazy-sounding songs is understandable in a man not long out of his teens. More surprising, maybe, is the revelation that Grint has never heard of My Bloody Valentine ("My Bloody ... who? I'm not aware of them"), and is equally unfamiliar with Neil Young ("Who's that?"). After all, he attends gigs quite regularly and used to frequent the NME Awards, where he met the Guns N' Roses' Slash ("Now he has got great hair").

"I went to see [Slash's post GNR band] Velvet Revolver, actually, which was ... an experience. I chatted to Slash and he was cool but quite, erm ..." He searches for the most apt adjective. " ... spaced out. Did he know who I was?" He laughs. "We didn't get that far. It was very brief. Very, very brief. We were standing on the fire exit stairs."

A shame because, had circumstances been more favourable, Grint would no doubt have sought advice from Slash given that he, too, would like to one day form a band. For the past 12 months, he's been learning to play the banjo and can now perform "half a song from Deliverance". Trouble is, no matter how accomplished he becomes, Grint seems too diffident, too nice, to front a group.

"I think you're right," he says. "I wouldn't be like ... Bono." Grint, one of the most famous young actors in the world, a man who has shared equal billing with such names as Michael Gambon, Maggie Smith and Julie Walters, lets his head flop forward and messes with his hair. "I'd be happiest standing at the back, strumming my little banjo where no one could see me." He grins. "Yeah," he says, "that would suit me fine."

• Harry Potter and the Half-Blood Prince is screened nationwide from 15 July

The symptoms: Whats on the patient's iPod

The Clash London Calling

Black Lips Bad Kids

The Specials Ghost Town

The Horrors Sheena is a Parasite

Sex Pistols Pretty Vacant

Oasis Champagne Supernova

The Streets Dry Your Eyes

Dead Kennedys Kill the Poor

Kings of Leon On Call

Does it Offend You, Yeah? We Are Rockstars

The prescription

The Ruts Staring at the Rude Boys

This old-school punk rabble-rouser was, unsurprisingly, his pick of the bunch. "I thought this was great, yeah. It reminded me of the Clash. Give me more."

Rakim Follow the Leader

One of the 80s' quintessential hip-hop cuts. "I've never heard of Rakim, but I'm assuming he is an old-school rapper. Either way, it's pretty decent. Nice beats."

The Twilight Sad Cold Days From the Birdhouse

My Bloody Valentine by way of Glasvegas. He approved. "I love Mogwai and this has that same quiet-loud thing going on. Wasn't sure about the vocals, though."

Neil Young Cortez the Killer

No longer could the Doctor allow the Patient to remain oblivious to Neil Young. "This is the first song of his I've heard. Not bad, but it shows its age a bit."

The record doctor will see you now ...

OMM's GP is now available to treat all-comers. Here are his diagnoses for some of the problems he has received so far by email ...

Q: I am a retired raver who has started to get cravings for some repetitive beats in my life again. As I'll never see 30 again, I'm quite taken with this balearic/cosmic craze that I keep hearing about, but don't want to make a fool of myself in my local record emporium. Who should I be buying and, more importantly, dancing to? Wroteforluck

A: Well, Wroteforluck, as anyone who has heard their remixes of Grace Jones's William's Blood and Friendly Fires' Paris will attest, the Belgium-based Aeroplane (Stephen Fasano and Vito Deluca) are the darlings of the psychedelic disco scene. Their own productions are released on Eskimo which, quite frankly, can do little wrong: witness Lindstrom and Prins Thomas's II, a mish-mash of prog rock, disco and Krautrock, and Daniele Baldelli's Cosmic Disco?! Cosmic Rock!, which explores space-age house music's 80s roots. Look out, finally, for Smith & Mudd, whose new album, Le Suivant, is suitably sun-kissed.

Q: My dad keeps buying insipid blues CDs and playing them at full volume on long car journeys. I bought him a Seasick Steve album for Christmas, which he loved. Can you point me in the direction of some authentic bluesmen? KittyCommando

A: Bluesmen don't come any more authentic than Robert Johnson and Blind Willie Johnson. The former's King of the Delta Blues Singers, first released in 1961, includes such tracks as Hell Hound on My Trail, on which he alludes to being hounded by the devil in an otherworldly howl. Insipid it is not. As for Blind Willie, beg, borrow or steal his Dark Was the Night, Cold Was the Ground, a wordless cry that reverberates down the years and which Ry Cooder once described as "the most soulful, transcendental piece in all American music". Overblown? Possibly - but you can see his point.